Today's long-haul communication systems are hybrids of optical and electronic components. For example, the repeaters detect light photoelectrically, amplify the resulting current electronically and then use the amplified current to drive a semiconductor laser which again converts the electrical signal back into an optical signal. The optical signal is then carried in an optical fiber to the next repeater in the system where the conversion from optical to electrical and back again to optical is repeated again.
In an all-optical transmission system, light, once generated, will be transmitted optically, amplified optically, and received by optical detection. There is no intermediate conversion from optical to electrical and then back to optical form. Direct optical amplification of an optical signal which results in the elimination of the electronic processing will enable optical communication systems to have repeaters which have higher bandwidths, are physically smaller, simpler in design, more efficient to operate and more economical to produce.
The performance of optical amplifiers is relatively unaffected by changes in the data bit rate and by the presence of additional channels at separate wavelengths, thus allowing for the possibility of upgrading an installed system to a higher capacity by changing only the equipment at the terminals.
Currently, research and development on semiconductor optical amplifiers is aimed at eliminating many of the optical to electrical and electrical to optical conversions which are presently required in optical communication systems.
One area which can present a problem is that of determining and/or controlling the output power of an optical amplifier. This is necessary because the gain of optical amplifiers can be affected by both environmental effects (i.e., changes in ambient temperature) and variations in system variables (i.e., changes in source wavelength and the polarization of the input signal). Another area of concern, when using optical amplifiers at repeater sites of optical communication systems is that of being able to send telemetry command signals to the optical amplifiers and having the optical amplifiers detect the telemetry command signals.
Currently, the power output of an optical amplifier is determined by diverting a portion of the generated optical power by means of an optical coupler and directing the diverted power to an optical detector. A primary disadvantage of this method is that a portion of the optical power generated by the optical amplifier is lost by the monitoring process. Moreover, it involves using a separate detector and a coupler.